The present invention relates to the production of high current density beams of electrons. More particularly, the present invention relates to an apparatus and method for extending the lifetime of a photoemissive electron beam generator and for providing temporally constant current production in the form of a high current density beam of electrons.
A prior art GaAs photoemissive electron beam generator is described in U.S. Pat. No. 4,460,831, issued July 17, 1984, entitled "Lase, Stimulated High Current Density Photoelectron Generator and Method of Manufacture," the disclosure of which is incorporated by reference. GaAs and other semiconductor photoemissive electron beam generators emit high current density beams of electrons in response to illumination by an appropriate laser or light-emitting diode. These beam generators are particularly suitable as electron sources for electron beam semiconductor lithography. Electron beam lithography systems can provide smaller feature size, and accordingly, greater feature density, on semiconductor chips than can be created with optical lithography systems. Present optical systems are limited to the production of features in the 0.8-1.0 .mu.m range, while electron beam lithography has created features below 0.5 .mu.m in width and is expected to provide features below 0.25 .mu.m. Features of this size would provide a substantially improved chip density.
Photoemissive electron beam generators are also suitable for electron microscopes, especially for those employed to inspect the highly resolved features on the semiconductor chips just described. The high brightness of the beam, and its imagibility from a few tenths of a micron to one micron or more, will allow high speed inspection of densely packed chips.
In order for an electron beam lithography system to be commercially viable as a machine for production runs of semiconductor chips, it would ideally provide a throughput that is competitive with that of conventional optical lithography systems. Currently, commercial production rates of optical systems are in the range of 40 wafer levels per hour. To achieve these throughputs and provide the desired high feature resolution with an electron beam lithography system, its electron beam must be very bright, i.e., have an exceptionally high current density and relatively small electron energy spread. The beam brightness is particularly important for a high throughput in an electron beam lithography system because the desired pattern is written sequentially. In contrast, high wafer level per hour throughput is a consequence in optical lithography systems of projecting all the features of an entire pattern simultaneously onto the wafer. Another important requirement for electron beam lithography systems is that the performance of the beam generator must be stable over time. The high brightness beam must be maintained throughout a commercially acceptable time period (preferably many months) in order to maximize the operating time and wafer throughput and thereby justify the cost of the relatively expensive equipment.
A GaAs crystal may be used as the electron source in a photoemissive beam generator that provides a high brightness beam. Such a crystal emits electrons in response to irradiation by visible and near-infrared incident light beam following "cesium-activation" by the deposition of small amounts of cesium and oxygen or fluorine in a prescribed manner on it surface. The term "cesium-activation," as used herein, refers to this method of activation. A preferred photoemissive surface has a GaAs crystal with cesium and either oxygen or fluorine, e.g., from nitrogen trifluoride (NF.sub.3), deposited thereon. In applications where such a semiconductor photoemitter or photocathode is sealed in a closed environment, such as in a image convertor tube, an equilibrium is established between the photosurface elements and the volume in the tube and its walls, whereby the rate of loss of activation elements from the photosurface is balanced by the arrival and redeposition of similar materials from the tube walls. Under these conditions, the resultant beam generator is relatively stable over time. However, in an application where this type of photocathode is used in an electron optical column which is constantly pumped to maintain a high vacuum, such as an electron beam lithography system, a deposition equilibrium is not established and the loss of activation elements from the photosurface is not balanced by redeposition This yields a net loss of the activation materials from the photoemissive surface, resulting in a degradation of the electron beam generating ability over time.
In order to counteract the degradation of a photoemissive surface in the non-equilibrium environments, it has been the practice of the prior art to periodically reactivate the photoemissive surface by first interrupting beam generation, redepositing cesium, and then restarting beam generation. See, e.g. previously cited U.S. Pat. No. 4,460,831. While the beam interruption required by this method is not a problem for some applications, it is commercially unacceptable in lithography systems where continuous operation and constant electron emission are required.
Accordingly, it is an object of the invention to provide an apparatus which provides a continuous stable high brightness emission of electrons over long periods, e.g., several months or more, from a semiconductor photoemissive electron beam generator.
Another object of the invention is to provide means for continual replenishment of cesium lost from a cesium-activated photoemissive surface of a photocathode.
A further object of the invention is to provide a method of reducing the desorption of activating elements from a photocathode surface.
These and other objects and features of the invention will be apparent from the following description and the drawings.